The most commonly observed leaf spot diseases of shade trees and shrubs in Minnesota include powdery mildew, anthracnose, and apple scab. Each of these diseases is described in detail in separate publications. In addition there are many other leaf spot diseases that occur on a wide range of native and ornamental trees and shrubs. Many leaf spot diseases have similar biology and therefore very similar management options. Information about some of these diseases is included below along with management information applicable to all leaf spot diseases of shade trees and shrubs.

Importance

Leaf spot diseases weaken trees and shrubs by interrupting photosynthesis, the process by which plants create energy that sustains growth and defense systems and influences survival. Fortunately, most leaf spot diseases affect only a small percentage of the tree's overall leaf area, and are a minor stress on the health of the tree. However, leaf spot diseases should be taken more seriously if they result in moderate to complete leaf loss two to four years in a row. Leaf loss during several consecutive growing seasons can result in reduced growth and increased susceptibility to opportunistic pests and pathogens.

Pathogens and susceptible plants

The majority of leaf spot diseases are caused by fungi, but a few unique diseases discussed below are caused by bacteria or other pathogens. Many of these pathogens are somewhat host specific and will only cause disease on trees of the same species or genus. Almost all trees and shrubs are susceptible to one or more leaf spot diseases.

Identification

Leaf spots come in a wide variety of shapes, sizes, and colors. Frequently leaf spot diseases are first observed on the lower and inner branches where humidity is higher and leaves are somewhat shaded. Spots occur randomly on the leaf surface because the pathogens that cause leaf spots are blown there by wind or splashed by rain or irrigation. Depending upon the pathogen, leaf spots may occur on the upper, lower, or both surfaces of the leaves. Leaf spots may be angular or rounded, raised or sunken, and have smooth or fringed edges. Colors are extremely variable, and can range from yellow to yellow-green to orange-red to light tan, brown, or black. Frequently a variety of sizes of leaf spots can be observed on one plant. The smaller leaf spots are younger infections, and the larger leaf spots are older infections. At the center of larger leaf spots, it is often possible to see signs of the pathogen such as fungal spores or spore-producing structures. Some leaf spot pathogens cause leaves to drop prematurely, resulting in early defoliation of the tree or shrub.

Biology

The pathogens that cause leaf spot diseases often overwinter in infected leaf debris. Some also infect buds and young twigs. During the growing season, wind and splashing rain carry spores of the pathogen to susceptible plant tissue and spread the disease throughout the canopy.

Most leaf spot diseases need either water on the leaves or very high humidity for a prolonged period of time (12 to 24 hours) to start an infection. Leaf spots often mature in one to two weeks. At this point each leaf spot produces spores or bacteria that can be spread throughout the canopy, starting a second set of leaf spots, or causing new infections on other plants. This cycle of infection and spore production repeats whenever weather conditions are favorable. As a result, in years with very high humidity or frequent rain events, leaf spot pathogens can spread throughout a tree or shrub's canopy resulting in severe disease.

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Unique forms of leaf spot diseases

Pathogens that cause leaf spots and shoot blight or canker

Many leaf spot pathogens are only able to produce symptoms in leaf tissue; however, some leaf spot pathogens can also cause blight or cankers of twigs. Blight refers to a progressive dieback of young, green shoots. Leaf spot pathogens that cause dieback of young shoots typically do not progress to infect the older woody branches.

Examples of leaf spot diseases that progress to leaf and shoot blight include Aschochyta blight on lilac and Venturia shoot blight on Populus species.

Cankers are infections of shoots and branches that start as round to oval discolored areas where the bark has been killed. As the infection progresses cankers girdle the stem resulting in wilt and death of any leaves above the canker. Branches infected with canker causing fungi often have discolored bark that may be cracked or oozing gums or dark-colored sap. Spore-producing structures often emerge along the margins or on the face of the cankered area.

Examples of diseases that result in both leaf spots and cankers in Minnesota include Septoria leaf spot and canker of poplar and Septoria leaf spot and canker of dogwood.

Management of pathogens that cause leaf spot and blight or canker should include the management practices listed in the management section below. In addition, trees and shrubs should be carefully scouted for discolored areas of bark. Any infected branches or shoots should be pruned out several inches below visible signs of the infection. Infected branches should be burned or buried.

Photo gallery of leaf spot/canker diseases

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Leaf rusts

Symptoms of leaf rusts include bright yellow, orange or red leaf spots. These leaf spots often produce an abundance of yellow, orange, red, or brown powdery spores that can be easily rubbed off and seen on a white tissue or paper towel. Spores may be produced in little blisters within leaf spots or may emerge from tiny cups or tubes on the lower surface of the leaf. In some hosts, leaf rust fungi also infect petioles, young green stems, and fruit.

Leaf rust diseases are caused by a group of related fungi. Some rust fungi only need to infect one host plant to complete its lifecycle. Other rust fungi require two different host plants. The most common examples of rust fungi that require two hosts in Minnesota are cedar apple rust and other Gymnosporangium rusts. These rusts infect one or more plants in the Rosaceae family (e.g., apple) and also infect species of Juniperus spp. Spores of rust fungi can be blown long distances and like many other leaf spot pathogens, rust fungi need long periods when leaves remain wet to start an infection.

In addition to the general management practices listed below, some rust resistant varieties are available for certain trees and shrubs. In addition rust infections can be reduced by avoiding planting two different hosts that are susceptible to the same rust fungus in close proximity to one another.

Photo gallery of leaf rust diseases

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Downy mildews

Downy mildew infections are first noticeable as light green spots on the upper side of the leaf that turn brown with time. Downy clumps of delicate white filaments and spores form on the undersides of leaves during humid weather. If the leaves are infected when young, they may become distorted.

Downy mildew is caused by a group of pathogens known as water molds or Oomycetes, which are related to algae. On trees and shrubs, downy mildew is often caused by the water molds, Peronospora spp. or Plasmopara spp. Water molds thrive in wet conditions and can be very problematic in rainy years. Downy mildew is rarely a problem in hot dry weather.

Many of the management practices listed below are effective in reducing disease severity of downy mildew. Most fungicides available to home gardeners, however, are ineffective against downy mildew pathogens.

Photo gallery of downy mildew diseases

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Bacterial leaf spots

Bacterial leaf spot diseases often start as small dark brown to black spots with a halo of yellow tissue surrounding each spot. In some bacterial leaf spot diseases, the center of the leaf spot will dry up and fall out, giving the leaf a "shot hole" appearance. If weather conditions remain favorable for disease, some bacterial leaf spots will grow together creating large black blotches on leaves or turning leaves completely black. Shoots, buds and occasionally flowers can also become black and blighted by bacterial leaf spot pathogens. Bacterial blight of lilac is a common bacterial disease in Minnesota.

Bacterial leaf spot diseases are most commonly caused by the bacterial pathogens Pseudomonas spp. or Xanthomonas spp. Bacterial plant pathogens often live on plant surfaces in low numbers without causing immediate symptoms. They can also travel long distances on moist air currents or be moved short distances on splashing rain and irrigation. When weather conditions are right, pathogen populations grow dramatically and cause disease.

Management of bacterial leaf spot diseases is similar to management of fungal leaf spot diseases except the majority of fungicide sprays will not protect the tree or shrub from a bacterial pathogen. Only sprays with copper or streptomycin as an active ingredient are effective in preventing bacterial diseases.

Photo gallery of bacterial leaf spot diseases

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Management of leaf spots diseases of trees and shrubs

Overall leaf spot diseases do not seriously harm your tree but many strategies can be done collectively to reduce the disease on the tree in following years.

Rake up and destroy fallen leaves before the first snowfall to eliminate locations where pathogens can survive to re-infect the plant the following growing season.

Do not overcrowd plants — use size at maturity as a spacing guide when planting.

Prune trees or shrubs to increase light penetration and improve air circulation throughout the canopy.

Wet conditions promote disease, so avoid or redirect lawn and landscape sprinklers that wet the lower canopy of the tree.

Reduce stress to your tree:

Water your tree throughout the growing season so that the top 6 to 8" of the soil is moist, especially during dry summer periods. Soil should be allowed to dry before watering again.

Maintain a 3 to 4" deep layer of mulch around your tree. Do not mound the mulch around the trunk of the tree but lay a flat layer with at least a 2 inch space between the mulch and stem to allow for air movement. Annually reapply mulch and inspect to ensure levels are maintained.

Do not fertilize trees and shrubs suffering from leaf spot diseases, unless it is recommended by a soil test to correct a nutrient deficiency.

Fungicides are not necessary unless a tree has been completely defoliated several years in a row.

Fungicides are protective and need to be applied before symptoms appear on the leaves. Proper timing of fungicide applications can vary depending on the biology of the disease. In addition, high-pressure spraying equipment is needed in order to get complete coverage of the canopy of large trees. Hire a professional arborist to treat leaf spot diseases in large trees.